Burst Mode optical receivers are typically located in the Optical Line Termination (OLT), or service node, or Local exchange, of an optical access network. Such an optical access network typically consists of a number of Optical Network Units (ONUs) located at a subscriber's premises and connected using a tree-like fibre plant to the OLT. The prior art has implemented burst mode receivers in which amplifiers are used whose output voltage is clamped at a fixed value once the input signal strength (voltage or current) exceeds a given value (so-called limiting amplifiers). This behaviour however is highly non-linear, which prevents the use of electronic equalization techniques to mitigate distortion in the input signal and certain classes of advanced modulation formats, which also require a linear receiver.
As all of the ONUs share the same fibre on the same wavelength, time division multiple access (TDMA) is used to transmit data upstream (from the ONUs towards the OLT). In TDMA, each ONU is assigned specific time slots during which it may send a burst of data upstream. As the bursts from different ONUs have undergone different amounts of attenuation while travelling towards the OLT, the signal at the OLT consists of a rapid sequence of bursts whose amplitude can differ greatly from burst to burst. To recover the transmitted data from this signal, a burst mode receiver is needed which quickly adapts its settings (gain and decision threshold) from one burst to the next. Similarly, in optical burst switched networks bursts may arrive at the receiver that have travelled along different paths, and hence have undergone different amounts of attenuation or amplification.
Today, there is significant Industry interest in increasing the bit rate over optical access networks towards 10 Gb/s and beyond. Burst-mode receivers operating at 10 Gb/s have been demonstrated, but still exhibit several problems:
1) Operation at sufficiently high bitrates implies that the properties of the optical fibre, such as chromatic dispersion, may severely distort the transmitted signal. This distortion can be compensated using techniques such as electronic dispersion compensation, but require that the optical receiver is linear with respect to the detected photocurrent. In today's, state-of-the-art, burst-mode receivers use limiting amplifiers which are highly non-linear and prevent the use of e.g. electronic dispersion compensation.
2) Burst-mode receivers today employ gain switching front-ends. This is needed to ensure that the burst-mode receiver does not distort the signal for strong input signals. For these strong input signals, the gain of the burst-mode receiver front-end is switched to a lower value. These gain switching front-ends enable fast (typically in a few nanoseconds) adjustment of the gain during the preamble at the start of each burst. However these gain switching architectures show severe problems for input signal strengths close to the switching points, for example, as late switching may occur resulting in significant numbers of errors or the loss of entire bursts.
3) A burst-mode receiver typically needs a control signal that indicates the start of a new burst. As it is not known a priori when a new burst will arrive, the burst-mode receiver detects the arrival of a new burst itself. This is done by detecting the transition of the input from its zero level towards the level of the incoming burst. However, if detected from the first rising edge of the burst, the detected moment of arrival of the new burst may be highly inaccurate. Indeed this first rising edge may exhibit a high amount of jitter as the transmitter is still turning on (e.g. in case of a directly modulated laser there may be a significant turn-on delay). This is problematic for any receiver circuitry that relies on an accurate detection of the start of a new burst.
A number of attempted solutions to these problems have been proposed in the prior art. For example, a first publication (S. Nishihara et. al., ‘A burst-mode 3R receiver for 10-Gbit/s PON systems with high sensitivity, wide dynamic range, and fast response’, IEEE Journal of Lightwave Technology, vol. 26, pp. 99-107, January 2008) describes the concept of a 10 Gb/s burst-mode receiver, that uses gain switching to enlarge its dynamic range. There are two main problems with the solution proposed in this paper:—
1) The burst-mode receiver disclosed in the paper uses gain-switching to reduce its gain from a high state to a low state for sufficiently strong input bursts. This is done by comparing the strength of the burst with a fixed reference. If the strength of the burst is greater than this reference, the gain of the burst-mode receiver is switched to its Low state. This gain-switching however has a serious problem, in that a burst with strength close to this reference, may cause gain switching to occur too late, for example during the data portion of the burst.
2) The burst-mode receiver disclosed in the paper uses a limiting amplifier to amplify the signals to a level that is compatible with a given logical format (such as e.g. current-mode logic). Such Limiting action is highly non-linear, thus preventing the use of electronic dispersion compensation to mitigate transmission impairments due to e.g. chromatic dispersion.
A second publication (T. De Ridder, P. Ossieur et. al., ‘A 2.7V 9.8-Gb/s burst-mode transimpedance amplifier with fast automatic threshold locking and coarse threshold extraction’, pp. 220-221, in Technical Digest International Solid-State Circuits Conference (ISSCC), February 2008) describes the concept of a 10 Gb/s burst-mode receiver front-end that quickly switches gain again by comparing the strength of the incoming burst to a reference. In variance from the first publication, an additional gain locking mechanism has been added in an effort to solve the problem of late gain switching. The described gain locking mechanism has two disadvantages however. First, it relies upon the use of a flip-flop. If this flip-flop exhibits metastability, again late gain switching may occur. Secondly, the described gain locking mechanism only works for a transimpedance amplifier front-end whose gain can be switched to a limited number of discrete gain settings. This excludes implementation of the described mechanism, whereby its gain should scale inversely with the input signal strength.
European Patent Publication number EP1357665 describes an automatic gain control method for a burst-mode optical receiver. U.S. Pat. No. 7,539,424 describes an automatic gain control method for a burst-mode optical receiver. However these patents do not solve the problems of automatic gain control accuracy, non-linearity of the burst mode receiver and do not implement any method to extract timing signals from the data bursts. European Patent Publication number EP1032145, assigned to NEC corporation, discloses an automatic gain control method for a burst-mode optical receiver. A means is disclosed to monitor the strength of the incoming burst, and based upon this strength to adjust the gain of a transimpedance amplifier so that the voltage signal outputted from said transimpedance amplifier is not saturated. However a problem with this approach is that it does not provide a method to ensure that the swing of the transimpedance amplifier equals a given reference, and that avoiding saturation in the transimpedance amplifier is not sufficient to ensure linearity. This is an important feature that is required for a linear burst-mode receiver, as the reference (and hence output swing) can then be optimized to ensure the linearity of the burst-mode receiver; for example by minimising total harmonic distortion.
An additional problem with today's state-of-the-art conventional linear optical receivers is that they rely on slow feedback automatic gain control loops with settling times exceeding hundreds of microseconds. Such long settling times are clearly not suitable for optical access networks or optical burst switched networks where the receivers need to respond to a new incoming burst during a few nanoseconds at the start (commonly known as the preamble) of each burst.
Further, referring to today's state-of-the-art, European Patent Publication number EP1935091 describes a method to derive a signal from a new incoming burst that indicates the start of this new burst. However, as the derived signal uses the very first rising edge of this new incoming burst, the signal that indicates the start of this burst is potentially inaccurate. Indeed at the start of the burst, the transmitter may not be fully switched on which results in a high amount of jitter at the start of the burst. US Patent Publication number US2009/0142074/A1 describes a method to detect the start of an incoming burst, and subsequently delays this signal. The detection of the start of the burst is performed using a relatively low-speed determinator. This time has to be added to the settling time of the first amplifier (2), which will result in unacceptable delays and jitter in detecting the start of the burst.
It is clear from the state of the art that there is a need to provide a solution to the problems associated with utilising gain switching and limiting amplifiers in burst mode optical receivers. It is further clear from the state of the art that there is a need to provide a solution to the problem of generating a signal that precisely indicates the start of a new incoming burst.
An object of the present invention is to provide a linear burst mode receiver to overcome the above mentioned problems.